The present invention relates to anti-lock control method and apparatus for preventing the locking of the automotive wheels of a running vehicle during the braking thereof and, more particularly, to an anti-lock control method in which an estimated vehicle speed is prevented from moving away from an actual vehicle speed due to an acceleration slip of an automotive wheel.
In a conventional anti-lock control apparatus for a vehicle, a controller including a microcomputer performs anti-lock control in such a manner that the mode of controlling the brake-liquid hydraulic pressure is determined on the basis of an electric signal indicative of a wheel speed detected by a wheel speed sensor. According to the detected mode, a hold valve, which is a normally-open solenoid valve, and a decay valve, which is a normally-closed solenoid valve, are selectively opened or closed to increase, hold or decrease the brake hydraulic pressure in order to keep the vehicle steerable and stable and to shorten the braking distance thereof.
The present inventor filed a U.S. patent application (Ser. No. 07/403375, now U.S. Pat. No. 4,984,164) as a related art of the present invention. An anti-lock control method described in the U.S. patent application will be hereinafter described for the purpose of a clear understanding of the present invention.
In the anti-lock control method, a reference speed Vr for pressure decrease judgment is set on the basis of the speed (hereinafter referred to as the wheel speed Vw) of each of wheels controlled by brake control units for the four wheels of a vehicle. An estimated vehicle speed is set, which follows the highest wheel speed (four-wheel select-high speed VwH) selected among the wheel speed Vw but whose decrease in its following the four-wheel select-high speed VwH is limited within a prescribed range of speed decrease rate. A first and a second threshold speeds VT1 and VT2 which follow the estimated vehicle speed Vv with prescribed speed differences therefrom are set (Vv&gt;VT1&gt;VT2). Statuses of pressure increase, pressure hold and pressure decrease, which are described hereinafter, are set on the basis of the comparison of the wheel speed Vw with the estimated vehicle speed Vv and with the reference speed Vr or the threshold speeds VT1 and VT2. A hold valve HV and a decay valve DV, which are solenoid valves, are opened or closed in prescribed control manners set for the statuses, to perform the pressure increase, the pressure hold or the pressure decrease.
FIG. 4 is a control time chart indicating the change in the wheel speed Vw and the brake hydraulic pressure Pw and the opening and closing of the hold valve HV and the decay valve DV in the anti-lock control.
The reference speed Vr is set as described from now on.
When the brake of the vehicle is not in operation during the movement of the vehicle, the brake hydraulic pressure Pw is not increased, the hold valve HV is open and the decay valve DV is closed. As shown in FIG. 4, when the brake is put in operation at a time point A, the brake hydraulic pressure Pw begins to be increased at the time point A, the brake hydraulic pressure rises sharply (in an ordinary mode) from the time point A, and the wheel speed Vw is thereby decreased. The reference speed Vr is set so that it follows the wheel speed Vw with a speed difference .DELTA.V therefrom so as to be lower than the wheel speed, and that the reference speed begins to rectilinearly fall with a speed decrease gradient .theta. of -1 G when the deceleration dVw/dt (negative acceleration) of the wheel has reached a prescribed threshold value of -1 G, for example, at a time point B.
Statuses shown in FIG. 4 are described from now on.
(Status 0)
The status 0 lasts from the time point A at which a brake switch is turned on by depressing a brake pedal, to the time point B at which the rectilinearly falling reference speed Vr is generated because the deceleration dVw/dt of the wheel speed Vw reaches the prescribed value of -1 G, for example. In the status 0, the hold valve is open, the decay valve is closed, and the brake hydraulic pressure in a wheel cylinder is increased because of the supply of the liquid from a master cylinder.
(Status 1)
The status 1 lasts from the time point B at which the rectilinearly falling reference speed Vr is generated, to a time point C at which the deceleration dVw/dt of the wheel speed Vw is judged to have reached a prescribed value -Gmax. In the status 1, the hold valve and the decay valve are not in operation.
(Status 2 (pressure hold))
The status 2 lasts from the time point C for the -Gmax judgement, to the earlier one of a time point (pressure decrease time point a) at which the wheel speed Vw becomes lower than the reference speed Vr, and a time point (pressure decrease time point b) at which the wheel speed Vw becomes lower than the first threshold speed VT1. AT the time point C, the hold valve is closed so that the brake hydraulic pressure is held. The status 2 terminates at a time point D at which the wheel speed Vw becomes lower than the reference speed Vr. However, if the wheel speed Vw becomes lower than the first threshold speed VT1 before the time point D, the status 2 terminates when the wheel speed has become lower than the first threshold speed VT1.
(Status 3 (pressure decrease))
The status 3 lasts from the time point D at which the wheel speed Vw becomes lower than the reference speed Vr, to a time point E at which the wheel speed Vw becomes lower than the first threshold speed VT1. At the time point D, the decay valve is opened so that the brake hydraulic pressure Pw begins to be decreased.
(Status 4 (pressure decrease))
The status 4 occurs when the anti-lock control is performing or has completed the second step thereof and the decrease rate VvG of the estimated vehicle speed Vv is smaller than -0.22 G. The status 4 lasts until one of three conditions (1), (2) and (3) mentioned below arises after the wheel speed Vw becomes lower than the first threshold speed VT1.
(1) A decay timer put in timing action at the time of the start of the pressure decrease becomes put out of timing action to prevent the pressure decrease from being excessively done.
(2) The wheel speed Vw becomes lower than the second threshold speed VT2.
(3) The wheel speed Vw is judged to be at a low peak, by detecting that the rates of the decrease and increase in the wheel speed Vw is within a prescribed range from -0.22 G to +0.22 G, for example.
(Status 5 (pressure decrease))
The status 5 occurs when the anti-lock control is performing the first step thereof or the rate VvG of the decrease in the estimated vehicle speed Vv is larger than -0.22 G. The status 5 lasts from the time point E at which the wheel speed Vw becomes lower than the first threshold speed VT1, to the earlier one of a time point F at which the wheel speed is judged to be at the low peak, and a time point F' at which the wheel speed Vw becomes lower than the second threshold speed VT2 as shown by a dotted line in FIG. 4.
(Status 6 (pressure decrease))
The status 6 occurs when the wheel speed Vw is lower than the second threshold speed VT2. The status 6 lasts from the time point F' to a time point F".
(Status 7 (pressure hold))
The status 7 begins when one of three conditions (1), (2) and (3) mentioned below arises.
(1) The wheel speed Vw is judged to be at the low peak in the statuses 4 or 5.
(2) The decay timer is put out of timing action in the status 4.
(3) The wheel speed Vw becomes higher than the second threshold speed VT2 in the status 6 (at the time point F").
The status 7 lasts from the above-mentioned beginning to a time point G at which the wheel speed Vw becomes higher than the first threshold speed VT1. If the wheel speed Vw does not become higher than the first threshold speed VT1 at or after the lapse of a prescribed time T1 in the status 7, the status is replaced by the status 4 so that the pressure decrease is performed again.
(Status 8 (pressure hold))
The status 8 lasts from the time point G at which the wheel speed Vw becomes higher than the first threshold speed VT1, to a time point H at which a status 9 exists.
(Status 9 (pressure increase start point))
The status 9 exists at the time point H at which the wheel speed Vw becomes higher than a speed Vv-.DELTA.V.sub.0 which is lower by a prescribed value .DELTA.V.sub.0 than the estimated vehicle speed Vv.
(Status 10 (fast build-up))
The status 10 lasts from the time point H at which the wheel speed Vw becomes higher than the speed Vv-.DELTA.V.sub.0, to a time point I at which a prescribed time T3 has elapsed since the time point H. In the status 10, the hold valve is repeatedly opened and closed so that the brake hydraulic pressure is increased relatively sharply.
(Status 11 (slow build-up))
The status 11 lasts from the time point I at which the fast build-up in the status 10 terminates, to a time point J at which the reference speed Vr is generated. In the status 11, the hold valve is opened and closed longer than opened, so that the brake hydraulic pressure is increased slowly.
(Status 12 (slow build-up))
The status 12 lasts from the time point J at which the reference speed Vr is generated, to the earlier one of a time point at which the wheel speed Vw becomes lower than the reference speed Vr, and a time point at which the wheel speed Vw becomes lower than the first threshold speed VT1. FIG. 4 shows that the status 12 terminates at the time point K at which the wheel speed Vw becomes lower than the first threshold speed VT1. However, if the wheel speed Vw becomes lower than the reference speed Vr before the time point K, the status 12 terminates when the wheel speed Vw has become lower than the reference speed Vr. At the time of the termination of the status 12, the status 4 or 5 arises again.
The anti-lock control method described above has an advantage that the anti-lock control can be appropriately performed to cope with various circumstances. However, a problem described from now on occurs if the estimated vehicle speed Vv increases far from the actual vehicle speed due to the acceleration slip of the driving wheel of the vehicle at the time of the start of the anti-lock control thereof.
When at least one driving wheel of he vehicle performs acceleration slip, the four-wheel select-high speed VwH rises sharply and the estimated vehicle speed Vv increases away from the actual vehicle speed along with the sharp rise in the four-wheel select high speed as shown in FIG. 5. When the speed of the wheel sharply decreases immediately after the acceleration slip thereof, a large speed difference occurs between the estimate vehicle speed Vv and the four-wheel select-high speed VwH because the decrease in the estimated vehicle speed Vv in its following the four-wheel select-high speed VwH is limited within a prescribed range of speed decrease rate. As a result, the threshold speeds VT1 and VT2, which are set on the basis of the estimated vehicle body speed Vv, are set to be higher relative to the four-wheel select-high speed VwH.
When the brake of the vehicle is put in operation during the sharp decrease in the driving wheel speed at a time point t0 shown in FIG. 5, the status 5 which a pressure decrease mode is brought about at the start of the anti-lock control because the wheel speed Vw has already become lower than the first threshold speed VT1 at the time point t0. The status 6 which is a pressure decrease mode begins at a time point t1 at which the wheel speed Vw decreases t0 be lower than the second threshold speed VT2. The wheel speed Vw becomes equal to the actual vehicle speed at a time point t2. The status 6 which is the pressure decrease mode lasts to a time point t3 at which the wheel speed Vw becomes higher than the second threshold speed VT2 because the threshold speed decreases along with the fall in the estimated vehicle speed Vv. The status 7 which is a pressure hold mode begins at the time point t3 because the wheel speed Vw becomes higher than the second threshold speed VT2. The status 8 which is a pressure keeping mode begins at a time point t4 at which the wheel speed Vw becomes higher than the first threshold speed VT1. The status 8 lasts to a time point t5 at which the wheel speed Vw becomes equal to the speed Vv-.DELTA.V.sub.0 lower than by the prescribed value .DELTA.V.sub.0 than the estimated vehicle speed Vv and at which the status 9 begins.
As described above, although the brake is put in operation at the time point t0, the vehicle is not braked since the time point t0 until the estimated vehicle speed Vv decreases to be nearly equal to the actual vehicle speed in the status 9, since the brake hydraulic pressure is not increased before the time point t0 and the statuses lasting from the time point t0 to the time point t5 are the pressure decrease mode and the pressure hold mode. This is the problem. In FIG. 5, the wheel speed Vw, which is caused due to the acceleration slip of the automotive wheel, can be regarded as equal to the four-wheel select-high speed VwH until the beginning of the status 9 because the brake hydraulic pressure for any Wheel of the vehicle is not increased.